package/python-tornado: only available for Python 3

package/python-tornado: only available for Python 3

Since the bump to 6.0.1, python-tornado has dropped support for Python 2
and now needs Python >= 3.5 (cfr. setup.cfg).

Apparently this is not checked by setuptools, and apparently
byte-compile is not done in the autobuilders, so there's not autobuild
failure.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Asaf Kahlon <asafka7@gmail.com>
Cc: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>